The present invention relates to a method of compensating delay in a linearization loop of a power amplifier, which loop comprises an I/Q modulator, one or more power amplifiers to be linearized and generating delay, and a feedback loop comprising an I/Q demodulator when the I/Q modulator and the I/Q demodulator derive an oscillation frequency from the same local oscillator.
Linearized amplifiers are needed in current digital wireless communications systems, for example, since it is required that the spectrum of a signal to be transmitted must not spread broader than the actual useful band. The spreading of a spectrum is caused by the nonlinearity of amplifiers and it generates interference to neighboring channels, for example. The linearity of amplifier stages depends on how they are biased and they can be classified according to linearity: a class A amplifier is the most linear but it has poor efficiency, while a class C amplifier, for example, has good efficiency but it is also highly nonlinear. Good efficiency is an important property in power amplifiers, and this is particularly emphasized in wireless communications devices whose battery capacity is limited. Hence, amplifiers that have good efficiency but are nonlinear and must be linearized are used.
A known method of linearizing a nonlinear radio-frequency power amplifier is cartesian feedback. In outline, its operational principle is as follows: the data to be transmitted is included in the baseband signals I and Q. These signals are conveyed to an I/Q modulator in which the signals are combined and modulated directly to a final frequency. A final-frequency signal is amplified by one or more nonlinear radio-frequency power amplifiers and conveyed to an antenna. The radio-frequency, amplified signal is sampled after the last amplifier stage by a directional coupler, for example. The sample signal is conveyed to an I/Q demodulator in which it is demodulated to baseband and the I and Q signals are separated from it. The baseband I and Q sample signals are finally summed to the actual I and Q signals. The summing generates predistortion of the I and Q signals, on account of which predistortion the nonlinearity generated in the power amplifiers is at least partly cancelled.
A problem in the arrangement described above is that the I/Q modulator and the I/Q demodulator both receive a local oscillator signal from the same source. On the other hand, the power amplifiers generate delay and the sampled I and Q signals return in a wrong phase. This distortion can be compensated by adjusting the phase of the local oscillator signal applied to the I/Q demodulator to correspond the phase of the I and Q sample signals. In the known solutions this phase adjustment is implemented by transformers, by special phase inversing circuits or a digital phase adjuster. The known solutions are similar in being space-taking and relatively expensive. Furthermore, the solutions are often complex and may require much manual work in production. Such solutions are thus rather poorly suited to small mobile communications devices produced in great numbers.
An object of the invention is thus to provide a method and an apparatus implementing the method so as to solve the above-mentioned problems. The objects of the invention can be achieved by a method of compensating delay in a linearization loop of a power amplifier, which loop comprises an I/Q modulator, one or more power amplifiers to be linearized and generating delay, and a feedback loop comprising an I/Q demodulator when the I/Q modulator and the I/Q demodulator derive an oscillation frequency from the same local oscillator, whereby the delay generated by the power amplifiers in the feedback is compensated by delaying a local oscillator signal applied to the I/Q demodulator, whereby the method is characterized in that the delay of the local oscillator signal applied to the I/Q demodulator is generated by an amplifier optimized to have a great delay, preferably by a small-signal amplifier.
The invention is based on the idea that delay is compensated by delay, not by phase adjustment. In other words, an arrangement which causes phase shift is replaced by an arrangement which generates delay in the signal branch applied to the I/Q demodulator of a local oscillator. An advantage of the solution is that the arrangement is fairly simple. In accordance with the invention, the delay of a local oscillator signal applied to the I/Q demodulator is generated by an amplifier optimized to have a great delay, preferably by a small-signal amplifier. The group delay, i.e. the delay, of the amplifier is preferably 5 to 10 ns or more. Since the local oscillator signal must usually be amplified before the I/Q demodulator in any case, the method of the invention can easily be used by optimizing an existing signal amplifier to have a delay. The advantage is that no additional phase adjustment circuits are needed.
The invention further relates to a linearization arrangement of a power amplifier, which arrangement comprises an I/Q modulator in which the baseband signals I and Q containing data are combined and modulated to a final frequency, one or more power amplifiers to be linearized and generating delay, by which power amplifiers a final-frequency signal is amplified and which signal is conveyed to an antenna to be transmitted after the amplification, and a sampling arrangement by which the amplified, final-frequency signal is sampled before the antenna, and an I/Q demodulator to which said signal sample is applied and in which the signal sample is demodulated to base-band and I and Q sample signals are separated from it, and a feedback in which the I and Q sample signals demodulated and separated from the signal sample are summed to the actual I and Q signals, and a local oscillator from which a local oscillator signal is conveyed to the I/Q modulator and the I/Q demodulator, whereby an arrangement is provided between the local oscillator and the I/Q demodulator, by which arrangement the local oscillator signal applied to the I/Q demodulator is delayed in order to compensate the delay generated in the power amplifiers, whereby the linearization arrangement is characterized in that the arrangement for delaying the local oscillator signal applied to the I/Q demodulator is an amplifier which is optimized to produce a desired delay to a desired frequency range, and that the amplifier is preferably a small-signal amplifier. In accordance with the invention, the arrangement for delaying the local oscillator signal applied to the I/Q demodulator is an amplifier which is optimized to produce a desired delay to a desired frequency range, and which amplifier is preferably a small-signal amplifier. By means of such an arrangement, the advantages provided by the method of the invention can be achieved by a simple structure which requires no more than few additional components and which is relatively inexpensive.